3 Answers
3

Since no one has stepped up yet I’ll try to give an answer to this. It’s mostly gathered from reading http://css3pie.com/documentation/product-comparison/ and googling around. I only have hands on experience with CSS3PIE. If you know any of the other projects feel free to edit this

CSS3PIE by Jason Johnston
Adds good support for Css3 border-radius, gradients, box-shadow.
My experience with this is that it works pretty well and has great support for border-radius. Problematic for hover effects (I would expect all items on this list suffer from this) but seem flawless when used on static html.
More info at http://css3pie.com/

ie-css3.js by Keith Clark
Adds support for a lot CSS3 pseudo-class selectors. Is extremely small since it feasts on other js libraries you include, like jQuery.
This could work well with other projects on this list like CSS3PIE.
More info at http://www.keithclark.co.uk/labs/ie-css3/UPDATE The ie-css.js project has been reborn here: http://selectivizr.com/

IE9.js by Dean Edwards
Works for IE6, 7 and 8. Make IE support transparent PNG, new CSS selectors like parent>child and :first-of-type, fixed positioning, max/min width/height support.
It's my impression a lot of work have gone into this over the years. Some reviews are really positive, some dismissive.
More info at http://code.google.com/p/ie7-js/

ecsstender by Aaron Gustafson
Works with IE6, 7 and 8. In a modular way adds support for CSS3 selectors, font-face, border-radius, box-shadow, colors and transforms.
Very well documented. More info at http://ecsstender.org/extensions

This article is a great intro to "border-radius.htc" but for the most part it's a list of IE-only CSS filters that can mimic CSS3 features. As a web developer I would like to not write a lot of old IE specific CSS code, but focus on standard CSS and have IE react to that, like "border-radius.htc" and "CSS3PIE" does.
–
mawtexAug 19 '10 at 9:47

Try Google Chrome Frame. IE users who install it will be able to browse your site in Google Chrome's renderer without switching browsers.

EDIT: You even can use a JS library provided by Google to show a message to users inviting them to install Google Chrome Frame. It's a one-click install, like Flash, does not require restarting the browser, and refreshes your page when it's done.

So, your suggestion is basically to somehow reach out to all the people who are going to visit the question poster's site in the future with IE6/7/8 and get them to install Google Chrome Frame before they visit? wouldn't it be easier to just get all IE users in the world to switch to Chrome instead?
–
NirOct 20 '10 at 8:45

@Nir I edited to explain how you can get your users to install GCF using Google's JS library. It's quite nice.
–
Dan FabulichOct 23 '10 at 2:30

even with one click installs (BTW, on Vista you need to Ok 5 security warnings to install Flash - so much for one click) I wouldn't dream of asking my users to install a browser plugin just so I can use rounded corners (and I suspet the only ones who will install are the one who really trust and like my product - that is, the ones who will buy even without the rounded corners).
–
NirOct 23 '10 at 22:05